1. Field of the Invention
This invention relates to acrylic polymer powder suitable for acrylic sol, an acrylic sol comprising the acrylic polymer powder and a plasticizer, and a molding obtained from the acrylic sol.
2. Statement of Related Art
Plastisol is matter obtained by dispersing resin powder and a filler in a plasticizer into a sol state, and the plastisol is applied by application or the like, and heated to gel into a molding. A resin industrially widely used for this process is polyvinyl chloride which is called polyvinyl chloride sol or polyvinyl chloride paste. The polyvinyl chloride sol composition is used in many fields for various purposes, namely as coating agents, impregnants, coking agents, etc. for automobiles, carpets, wallpaper, floors, etc.
On the other hand, moldings obtained from polyvinyl chloride sol compositions have the defect that, when disposed of by combustion, they strikingly damage incinerators due to the generation of hydrogen chloride gas from polyvinyl chloride. Furthermore, on recent environmental problems, polyvinyl chloride is not only a cause of acid rain but a causal substance of the destruction of the ozone layer of the earth. Thus, the appearance of plastisol free of these defects and substituting for polyvinyl chloride compositions has been desired in various commodity fields.
Under the demand, as a plastisol not generating hydrogen chloride gas at the time of combustion, acrylic sols using an acrylic resin are proposed. For example, an acrylic sol using acrylic polymer particles of a uniform composition is proposed (see GB1516510A corresponding to JP51-71344A, especially claims), but the acrylic sol has the problem that when a general purpose plasticizer such as dioctyl phthalate is used, the solubility of the particles in the plasticizer is high and the rise in viscosity of the acrylic sol occurs in several minutes after the mixing, which leads to the impossibility of film formation, and thus the acrylic sol can scarcely be used in practice. For improving the film formability and storage stability of acrylic sol, a process to copolymerize a monomer component having low compatibility with the plasticizer in an acrylic polymer is proposed (see EP533026A corresponding to JP5-279539A, especially claims), but such a sol composition has the problem that when used, the plasticizer is apt to bleed out onto the surface of the film obtained.
Plastisol compositions as acrylic sols in the state of dispersion in a plasticizer in which plastisol compositions the secondary average particle size is in the range of 0.1 to 500 μm are proposed (see GB1516510A corresponding to JP51-71344A, especially claims; JP54-117553A, especially claims). However, when an acrylic sol containing an acrylic polymer whose secondary average particle size is very small is used, the initial viscosity of the sol is high because of the large particle area per volume of the acrylic polymer particles, dilatancy is apt to occur, the fluidity of the sol is lowered, and troubles such as the uneven thickness or faintness of the film at coating, and the unevenness of spray pattern at spray coating take place. For getting such an acrylic sol containing an acrylic polymer whose secondary average particle size is very small to have a practical viscosity, there arises a necessity, e.g., to increase the amount of the plasticizer or add an organic solvent for dilution. As a result, such a problem that the plasticizer bleeds out onto the film surface, the mechanical strength of the film is lowered or the organic solvent added remains in the film and spoils the film formation is apt to occur.
On the other hand, when an acrylic sol containing an acrylic polymer whose secondary average particle size is large is used, the tendency that the viscosity is lower than that of the acrylic sol containing an acrylic polymer whose secondary average particle size is very small is seen, but there is a problem that since the uniformity of the acrylic polymer in the acrylic sol is poor and thus fish eyes are formed to prevent the formation of smooth film, and moreover, there is also a problem that since it takes a long time for the gelation of the film by heating, the productivity of products coated with the acrylic sol is apt to be lowered.
Although an acrylic sol of low viscosity containing a mixture of acrylic polymer particles having an average particle size of 0.1 to 2.0 μm with acrylic polymer particles having an average particle size of 3.0 to 50 μm is proposed (see JP 8-73601A, especially claims), there are drawbacks that a troublesome step of making a dried resin (secondary particles) into the desired particle size distributions by classification or pulverization and mixing the resulting two kinds of acrylic polymer particles is needed, and moreover the particles are destroyed by shearing force generated in the mixing with the plasticizer, etc. and thereby the stability of the sol is lowered.
As stated above, there was the problem that when a general purpose plasticizer such as dioctyl phthalate is used in acrylic sol, the solubility of the particles in the plasticizer is high and the rise in viscosity of the acrylic sol occurs in several minutes after the mixing, which leads to the impossibility of film formation, and thus the acrylic sol can scarcely be used in practice. Further, when, for improving the storage stability of an acrylic sol composition, such an acrylic sol composition that a monomer component having low compatibility with the plasticizer is copolymerized in the acrylic polymer contained therein is used, the acrylic sol composition had the problem that the plasticizer is apt to bleed out onto the surface of the film obtained. Thus, in the case of the use of acrylic polymer particles, storage stability and plasticizer retention (bleed-out resistance) after film formation are incompatible, and it was impossible to satisfy both properties in polymer particles of uniform structure.
Thus, an acrylic sol composition using particles of core shell structure is proposed in the above-mentioned EP533026A corresponding to JP5-279539A, especially claims, and an acrylic polymer wherein an acid is made to be contained in the polymer skeleton is used therein. However, the polymer proposed in the document is low in compatibility with plasticizers, and when a plasticizer low in polarity such as a phthalic ester is used, the state of plasticization gets poor and good film cannot be obtained. Further also in U.S. Pat. No. 5,441,994A corresponding to JP6-322225A, especially claims, a plastisol composition using particles of core shell structure is proposed. Although the word, particles of core shell structure, is used, the word therein means particles obtained by preparing particles of uniform structure and hydrolizing them with an alkali to convert the ester groups at the surface parts of the particles to carboxyl groups. Therefore, the thickness of the shell parts is extremely thin, and the shell parts substantially occupy only about 1% or less of the volume of the particles. Thus, the effect of the improvement of storage stability expected as a role of the shell parts is extremely low. The shell parts introduced by alkali hydrolysis has a very high acid value, and thus their compatibility with the plasticizer is extremely low and film formability is strikingly lowered. Moreover, such structural particles as above having shell parts of a high acid value are apt to take aggregative structures consisting of the particles in the plastisol, and as a result, there arises a tendency that the viscosity of the plastisol composition at low shearing rates becomes higher and workability is lowered.
Other examples of plastisol compositions using core shell structural particles are proposed in JP7-233299A, especially claims and JP8-295850A, especially claims. Therein, by using a core shell polymer consisting of a core part basically showing compatibility with plasticizers and a shell part showing incompatibility with plasticizers, very fundamental performances (storage stability of a low level, plasticizer retention of a low level, etc.) are realized. However, in order to put a plastisol composition to industrially practical use, extremely high performances [storage stability of a high level, plasticizer retention of a high level, mechanical performances (tensile strength, tensile elongation, etc.) of high levels, etc.] are required. In that point, the polymer proposed in the above documents are unsuitable for industrially practical use because the balance between a plasticizer and it in compatibility is not optimized and any of storage stability, the plasticizer retention of the film and the flexibility of the film is in a low level.
In JP8-295850A, especially claims and JP9-77950A, especially claims, an acrylic sol is proposed using acrylic polymer particles comprising component (a) consisting of a specific monomer having good compatibility with plasticizers and component (b) consisting of a specific monomer having poor compatibility with plasticizers, the constitutional ratio of component (a) being decreased in a multistage way or continuously toward the outmost layer from the center of each acrylic polymer particle, and the constitutional ratio of component (b) being increased in a multistage way or continuously toward the outmost layer from the center of each acrylic polymer particle. However, the acrylic sol using the acrylic polymer particles has a problem that since the shapes and/or properties of the acrylic polymer particles are not clearly defined therein, compatibility between the surface layer of the particles and the plasticizer is low and the diffusion of the plasticizer into the secondary particles is poor, fish eyes resulting from the remaining secondary particles are formed during film formation and smooth film cannot be formed, and therefore, there is a case where practically undesirable results occur.
Although various trials have been made for making storage stability, plasticizer retention, film flexibility, etc., which are the most fundamental properties of plastisol, stand abreast, it is the present state of things that all these plastisols are in low levels and have not reached an industrially practical level, probably partly because the shapes and/or properties of acrylic polymer particles, etc. used are not clearly defined therein.